Contemporary powerboat hull design favors a deep V configuration to give a smoother ride through waves, however, the V hull design has the effect of reducing the boat's effective beam dimension as the boat, in increasing its speed, rises to a planing position, which position considerably reduces the boat's ability to resist banking caused by steering forces and/or uneven loading.
All pleasure powerboats have steerable propulsion apparatus or propulsion apparatus plus a stern rudder which causes said boats to bank in the direction the helm (i.e., the pilot's wheel or the tiller for steering the boat) is positioned off center. Boats may employ an inboard engine to drive a propellor with a fixed longitudinal drive shaft along with a separate (conventional) stern rudder to steer and bank the boat in the direction of the turn. Or they may combine the propulsion, steering, and banking functions into an outboard drive by angling the propellor thrust to port or starboard to steer the boat. A very popular mechanism of this kind is an inboard engine powering an outboard drive. This is called an IO drive. A less-frequently employed propulsion mechanism pumps sea water rapidly through fore and aft ducts on the bottom of a boat. The resulting jet and its thrust can similarly be diverted to steer and bank the boat. Herein, (1) the steerable propulsion apparatus and (2) the propulsion apparatus plus a stern rudder which options (1) and (2) serve as the two modes of propelling and directing the boat are referred to as the "driving and steering unit." As used herein, the term "driving and steering unit" excludes the helm, electronic controls, and cables or links which move the fin-rudder or move the steerable propulsion apparatus or move the stern rudder.
Over the years, the height of most pleasure powerboats above the resting waterline has increased, while the portion of the hull below the resting waterline has not. An IO drive (or, alternatively for instance, the aft skeg and separate conventional stern rudder of a straight-through drive), will enable the stern of a boat to resist making leeway from the force of a wind component on either side of the boat's course. However, the high topsides of the majority of contemporarily-designed boats cause their bows to make substantial leeway from the force of a side wind component.
To maintain a heading under the above wind conditions, the helm must be positioned to the right of center in order to compensate for a starboard wind component, and to the left of center in order to compensate for a port wind component. The offset of the helm from center will cause the boat to bank undesirably towards the wind direction in order to maintain a selected heading. Said banking projects a flat surface of a V-bottom boat toward oncoming waves, which results in severe pounding. In fact, a V-bottom boat will give a smoother ride in waves seen to be coming from a ten o'clock to two o'clock direction (60 degrees either side of the boat's heading) when the boat is caused to bank about 5 degrees to leeward.
The prior art discloses one device for powerboats up to 25 meters in overall length with means to cause a boat to maintain an average level attitude about its roll axis, by employing port and starboard trim tabs horizontally hinged to the boat's lower transom. Said tabs can be independently moved downward into the fast-passing water to exert an upward leveling force on either the port or starboard side of the boat. This transom double trim-tab concept is only marginally effective in that it does not prevent the leeway drift of the bow, nor does it utilize the practically unlimited force of the boat's driving and steering unit to level the boat or alter the boat's angle of bank or list.
On larger power boats, very powerful large-area, generally horizontal port and starboard fins called horizontal stabilizers are employed. Their angle-of-attack about a transverse axis is automatically and continuously adjustable. Their primary function is to resist the periodic rolling motion of the boat caused by waves, with a secondary function of providing a level attitude about the boat's roll axis.